Hard disk drives are at their most vulnerable moment when they write data to a track, because if they miss, data on nearby tracks may be corrupted. One classic source of such harmful events has been collisions between a read-write head and a dust particle on the rotating disk surface accessed by the read-write head, which is somewhat akin to an automobile driving over a rock or through a pothole, and ending up off track. These situations can often be determined to have happened by analyzing the Voice Coil Motor signal, in particular a component of it known as the Back Electro-Magnetic Force (EMF).
However, the problems faced today are much more serious than finding out what has already occurred from a collision. Rapid Off Track following is a very fast response by the head stack assembly to “popping phenomena” such as a non-linear release of stress-strain between two dissimilar metallic components under heat stress. This has several potential causes, slippage between plates making up the head stack assembly, the voice motor assembly and/or the cover assembly. These Rapid Off Track events occur in a very short time interval with a very sharp shock wave through the entire head stack assembly. They are by far the fastest mechanical disruptions that are known to adversely affect the operations of a hard disk drive. They are too fast to be handled by an interrupt handler for a standard microprocessor. They must be noticed before they destroy data on nearby tracks during write operations. And worse yet, they cannot be predicted beforehand.
Traditional mechanical shock sensors for hard disk drives typically track events below their self-resonant frequency of 20-30 K Hertz (Hz). These sensors cannot respond to disturbances in less than 30 microseconds, which is too late to prevent these disruptions from damaging the data of the nearby tracks of the hard disk drive.